1. Field of the Invention
The present invention relates to an optical pickup apparatus for reading a recorded signal from a track of a recording medium.
2. Description of the Related Art
In recent years, it have been planed in the HDTV (high definition television) art to digitalize a HDTV signal and record and reproduce it in an optical disk. The processing of the HDTV signal requires a very high transfer rate e.g., 50 Mbps (bps: bit per second), more than 40 times that of an optical compact disk (1.2 Mbps) as it is used.
In a conventional recording and reproducing method for an optical disk, information is recorded on a track spiralling from its inner to outer of the optical disk by using a single recording light beam and then, in case of reproducing it, the recorded information signal is read by using a single reading light beam. There is generally used a three-light-beam method for properly tracing the track by the reading light beam in which two satellite light beams are utilized for the tracking control.
FIG. 1 shows an arrangement of light spots formed by three light beams on the optical disk in the three-light-beam method in which recorded signals (pits) PT are read out of a central track 15 between the adjacent tracks 14, 16 formed on the optical disk. The pits are represented by crosshatching. A light beam is divided into three beams by a diffraction grating (not shown) and the three beams form the three light spots 11-13 on the optical disk. The light spot 11 is shifted and placed at an inner side by a 1/4 track pitch with respect to the track 15 in the optical disk (it is adjacent to the track 14), and the light spot 12 is in the center of the track 15 and the light spot 13 is shifted and placed at an outer side by a 1/4 track pitch with respect to the track 15 in the optical disk (it is adjacent to the track 1).
FIG. 2 shows diffraction patterns formed on a photodetector D by the reflected lights from light spots shown in FIG. 1. Namely, the reflected light from a track or pit within the light spot 11 of the optical disk forms a circular pattern 11a on an optical receiving element 17 of the photodetector D. The reflected light from a track or pit within the light spot 12 of the optical disk forms a circular pattern 12a on a quadrisected optical receiving element 18 of the photodetector D. The reflected light from a track or pit within the light spot 13 of the optical disk forms a circular pattern 13a on an optical receiving element 19 of the photodetector D. Since a cylindrical lens 8 is used in a detecting optic system as shown in FIG. 3, a photodetector D including the optical receiving elements 17-19 is disposed at a non-image formation position IV shifted from an image formation position IP of the pit of the optical disk 34 caused by an objective lens 33. Therefore, the circular diffraction patterns formed on the optical receiving elements 17-19 of the photodetector are larger than sizes of pit images on the image formation position IV obtained by a product both of the pit diameter and the transverse magnification of the objective lens, respectively. Furthermore, the optical receiving elements 17-19 of the photodetector a photodetector have a large area in such a manner that each element receives the moving circular light spot caused by the driven objective lens in the pickup apparatus.
A difference signal between the outputs obtained from the optical receiving element 17,19 of the photodetector becomes zero when the light spot 12 is on the center of the track 15. On the other hand, the difference signals during the shifted light spot positions of inner and outer sides of the track 15 become large with an inverse polarity respectively. It is therefore possible to control such that the position of light spot 12 is always on the center of the track 15 by driving the objective lens in the basis of the difference signal. The recorded signal is probably read on the basis of output signals supplied from an optical receiving element 18 of the photodetector receiving the reflected light from the light spot 12. In this way, a three-light-beam method utilizes only one of three light beams for reproducing the recorded signal information.
The three-light-beam method of tracking control is intended to keep the light spot at the center of the track in optical disk in such a manner that the circular diffraction pattern on the optical receiving element of the photodetector is moved by the objective lens. It is necessary to enlarge the area of the optical receiving element of the photodetector in order that the diffraction pattern moves out of the optical receiving element of the photodetector in the tracking control.
There is also known the push-pull method as a conventional tracking control method other than the three-light-beam method. In the push-pull method, a photodetector having a bisected optical receiving element is used. As shown in FIG. 4, the photodetector having bisected optical receiving elements 65, 66 is disposed at a non-image formation position IV shifted from an image formation position IP of the pit of the optical disk 34 caused by an objective lens 33. As shown in FIG. 5, when a light spot 13 is formed at a center of the track 15 on an optical disk, as shown in FIG. 6 the bisected optical receiving element 65, 66 receives a diffraction pattern 67 with an intensity distribution (hatching) caused by the zeroth and first order diffraction lights reflected from the pit within the light spot 13. The optical receiving elements 65, 66 have an area larger than the size of light spot 67 of the zeroth order diffraction light spot. The light spot position on the optical disk is controlled in such a manner that difference between the outputs supplied from the optical receiving elements 65, 66 is kept constant and at the same time the recorded signal in the optical disk is read on the basis of the diffraction patter. The push-pull method of tracking control method utilizes only one light beam for reading the recorded signal form the optical disk.
In order to raise the transfer rat in such methods above for reading the recorded signal by using a single light beam, it is necessary to speed up the rotation of the optical disk or make a high density of the recorded signals higher. However, there are problems in the speed up of the disk and the high density thereof. This is because the rotation speed of the optical disk is limited at about five times as it is, since it is difficult to control precisely the objective lens of the pickup device in response to the focus error signal caused by an eccentricity, warp and so on of the optical disk. Furthermore, the density of the recorded signals is limited at twice as it is even when a light beam with a short wavelength e.g., a blue light is used for recording the information signals. Even if both the speed up of the disk's rotation and the high recording density thereof are preformed, the limit of transfer rate is about ten times as it is at most.
On the other hand, there is a method for reproducing information from an optical disk by using three light beams for the HDTV which is presented in the general meeting 1993(20-1) of the Television Society. This method improves the transfer rate by means of the irradiations of three light beams to three adjacent tracks are preformed at the same time to read collectively the recorded signals in the three track. The optical receiving elements of the photodetector are disposed at the non-image formation position for the optical disk in the reading optics, in which the diameter of each light spot on the optical disk is converged to the limit of the diffraction and then, the recorded information included in the reflected light is read as one channel signal. It is necessary for the reading light beam to properly tracing the track in this method. Therefore, the three-light-beam method of tracking control as shown in FIG. 7 is utilized in this method.
As shown FIG. 7 of the method for reproducing information from an optical disk by using three light beams for the HDTV, light spots 21-23 are formed on the optical disk. The light spot 21 is shifted and placed at an outer side of the track 24 in the optical disk, and the light spot 22 is in the center of the track 25, and the light spot 23 is shifted and placed at an inner side of the track 26 in the optical disk.
FIG. 8 shows diffraction patterns formed on a photodetector D by the reflected lights from light spots shown in FIG. 7. Namely, the reflected light from a track or pit within the light spot 21 of the optical disk forms a circular pattern 21a on an optical receiving element 27 of the photodetector D. The reflected light from a track or pit within the light spot 22 of the optical disk forms a circular pattern 22a on a quadrisected optical receiving element 28 of the photodetector D. The reflected light from a track or pit within the light spot 23 of the optical disk forms a circular pattern 23a on an optical receiving element 19 of the photodetector D.
This method utilizes the photodetector similarly to the above three-light-beam method of tracking control method, in which the intensity difference between the reflected light spots 21a, 23a caused by the light spots 21, 23 are used for controlling the position of light spots. In this case, since the recorded signals of the track 24, 26 are read from the positions of reflected light spots 21a, 23a through the optical receiving element 27, 29, the transfer rate is improved.
However, the number of light spots have to be increased in order to improve the transfer rate in the particular method for reproducing information from an optical disk by using three light beams for the HDTV. As a result, it is a drawback to complicate the construction of the optical pickup apparatus using such a three-light-beam method.